1. Field of the Invention
The present invention is generally related to a coupler used for connecting two shafts together in torque transmitting relation and, more particularly, to an elastomeric coupler that exhibits two or more different rates of stiffness in response to different magnitudes of torque being transmitted by one shaft to another while allowing the engine mount system to adequately isolate the vibration of a motive source, such as an internal combustion engine.
2. Description of the Prior Art
It is common and well known to use elastomeric couplers to transmit torque from one shaft to another. The advantage of using an elastic material as a coupler relates to the ability of an elastic coupler to reduce vibration and noise at the connection between the two shafts. As one shaft rotates with a slightly different speed characteristic than the other shaft, the two shafts can exhibit slight degrees of rotation relative to each other. The elastic material is able to compress to accommodate this relative movement. Another advantage of using elastic couplers is that slight degrees of misalignment can be accommodated by the deformation of the coupler.
U.S. Pat. No. 3,837,179, which issued to Barth on Sep. 24, 1974, describes a flexible coupling in which the jaws of one flange are normally interdigitated with the jaws of the other flange to form substantially cylindrical chambers for reception of elastically deformable cylindrical inserts which transmit torque between the flanges when one flange rotates to drive the other flange. At least one of the flanges has a hub and a rim which is separably connected to the hub by screws and can be moved, upon removal of screws, with the respective jaws in a direction away from the jaws of the other flange to thus permit convenient withdrawal of the inserts. The inserts are connected to a split ring which can pass over the hub when the latter is separated from the rim.
U.S. Pat. No. 4,037,431, which issued to Sugimoto on Jul. 26, 1977, describes a coupling device that is used in a one way rotating drive. The coupling device is used for transmitting rotation torque in only one direction from the drive member to a driven member and has first and second flange members and an elastic circular compression member. The compression member has projections on the periphery which are grabbed by claws on the flange members. There are several pairs of claws, one claw of each pair being attached to the first flange and the other claw being attached to the second flange. The first and second flanges are connected to the drive and driven members, respectively. As the drive members rotates in its only rotating direction, torque is transmitted between the paired claws via the elastic projections between the paired claws. The elastic member also has smaller projections which extend between the claws of adjacent pairs and are therefore free of compression during rotation.
German patent 2439558 which issued to Schonenberg, discloses a flexible coupling for rotating shafts. The flexible coupling has two similar halves with claws that interlock. It also has a shaped elastic damping ring. The shaft coupler consists of two similar halves with claws in a circular pattern that keep the two mating faces at a prescribed clearance from one another. An elastic damping ring fits in the clearance space. The advantage claim for the design is that the damping ring is small, compact, of low weight and low cost of manufacture. There is a common inner ring for the two coupling halves with the elastic element held in the region of the claws by two cams. The elastic element is shaped in both axially directions to fit into the coupling halves. The inner surfaces of the claws are flat, but are cylindrical on the outer section in the form of a reinforcing tooth. Alternatively, the elastic element can be barrel shaped with curved facing surfaces.
German Patent 2609008, which to issued to Ward, describes a flexible shaft coupling with barrel shaped pressure blocks. The flexible shaft coupling ring consists of an even number of equally spaced barrel coupling blocks. The blocks are joined by alternative axially offset connecting webs. A circular assembly of compression blocks is thus obtained. The flanged coupling sleeves which are attached to each shaft and which have an equal number of location pockets for the flexible compression elements of the coupling ring are provided. The shape of the coupling ring allows a particular compact construction of the flanged shaft sleeves. Bigger shaft diameters can therefore be coupled with a given size coupling of this type.
U.S. Pat. No. 5,720,638, which issued to Hale on Feb. 24, 1998, describes an engine driveshaft coupler for a personal watercraft. A jet propelled watercraft has a coupling assembly to couple an engine crankshaft to a jet pump impeller shaft. The coupling assembly can accommodate substantial engine crankshaft vibrations yet effectively isolates the jet pump impeller shaft from transverse movement. The coupling assembly includes an engine crankshaft coupling head, an intermediate coupler, an impeller shaft coupling head and two elastomeric islolators positioned between each of the coupling heads and the intermediate coupler. The intermediate coupler is supported exclusively by the elastomeric isolators, and is allowed to tilt transfers through the rotational axis of the intermediate coupler to accommodate engine crankshaft displacement. The coupling assembly is practical for personal watercraft because although elastomeric isolators wear or shred quickly in the presence of transverse misalignment, elastomeric isolators provide significant durability in the presence of a reasonable amount of angular displacement. The coupling assembly allows the engine to be soft mounted to the hull of the watercraft and therefore significantly reduces engine noises resonating from the watercraft hull.
In many applications where torque is transmitted from a driving shaft to a driven shaft, there are severally goals which the designer typically attempts to accomplish. For example, during the transmission of low magnitudes of torque between the driving and driven shafts, it is often desirable that an elastomeric coupler exhibit a relatively low magnitude of stiffness. This low stiffness allows the elastomeric coupler to be deformed readily to absorb vibrations which can occur during periods of low torque transmission. For example, if the driving shaft is an output shaft from an internal combustion engine, the operation of the engine at idle speed can cause the driving shaft to rotate at varying speeds which are affected by the firing sequence in the cylinders of the engine. Upon each ignition of the gaseous mixture in a cylinder, a pulse of torque is applied to the driving shaft and this is then followed by an absence of a torque pulse until the next cylinder fires. If the elastomeric coupler is too stiff, the vibrations and noise resulting from the varying speeds of the driving shaft will be transmitted through the coupler to the driven shaft. Therefore, it is desirable to have a softer coupler, with a lower magnitude of stiffness, during periods when the engine is operating at idle speed. However, when the driving shaft is transmitting relatively high magnitudes of torque to the driven shaft, it is desirable to have a relatively stiff coupler which transmits the torque from the driving to the driven shafts without being overly compressed. Typically, a soft coupler can be over compressed when high magnitudes of torque are transmitted from the driving shaft to the driven shaft. Therefore, it can be seen that two significantly different characteristics of couplers are usually desirable to accomplish the contradictory goals of having a soft coupler with a low stiffness for low speed operation and a stiffer coupler for high speed operation when higher magnitudes of torque are transmitted from the driving shaft to the driven shaft.
The stiffness of a material is generally defined as its ability to resist deformation under stress. The modulus of elasticity is the usual criterion of the stiffness of a material. The modulus of elasticity, in either tension or compression, is the constant which expresses the ratio of unit stress to unit deformation for all values of unit stress not exceeding the proportional limit of a material. This term is also sometimes referred to as the coefficient of elasticity or Young's modulus. Since the elastomeric material used to manufacture a coupler will exhibit a constant modulus of elasticity, its stiffness will typically also be constant. Therefore, the soft elastomeric material that is particularly advantageous during low speed operation will be disadvantageous during periods when high magnitudes of torque are transmitting from the driving shaft to the driven shaft. Conversely, a coupler with a high degree of stiffness which is particularly advantageous during high speed operation, when significant magnitudes of torque are being transmitted, will be disadvantageous at low speed operation when a softer coupler is desirable.
It would therefore be significantly advantageous if a coupler could be developed which exhibits different rates of stiffness at different magnitudes of torque.